SD (Secure Digital) and MMC (Multi-Media Card) are the same physical format, if I remember right. The difference is that SD has some bits that allow for copy control in certain devices (which doesn't apply to cameras).

All I ever see in stores now are SD cards (and in fact, asking your average Best Buy drone about MMC may net you a deer-in-the-headlights look), so just get that.

For best results make sure you get a high-speed one, like Sandisk Extreme II, III, IV or Lexar's equivalent. Not much of a price premium to make sure you get a good card if you shop around.